1. Field of the Invention
This invention relates to systems for measuring the transmittance or reflectance of thin sheets, such as photographic films or other transparencies, having images and other information thereon. In a primary application, the invention relates to scanning systems where the transmittance or reflectance of a whole sheet is measured by scanning a spot of light throughout the sheet and measuring the amount of light transmitted or reflected at each point of the scan.
2. Description of the Prior Art
The light transmittance of a sheet is defined as the ratio of the intensity of light passing through a point on the sheet to the light incident on that point. Similarly, the reflectance of the sheet is defined as the ratio of the intensity of light reflected from a point to the intensity of light incident on that point.
It is often desirable to measure the transmittance or reflectance at an array of points along the sheet. Scanners, i.e., machines for automatically making these measurements, have been known and used in the past. They have wide application in the processing of images in medical analyses, graphic arts, and long distance communications.
A major problem in measuring transmittance of a sheet is collecting the light transmitted through the sheet. Several things can happen to light when it interacts with the material in the sheet. One is that the light can pass through the sheet without interacting at all. Another is that the light can be scattered or changed in direction, but still pass through the sheet. Still another possibility is that the light can be absorbed and transfer its energy to the material.
In measuring transmittance, if only the light that passes through the sheet without interacting is used, the resulting ratio of transmitted to incident light is called the specular transmittance. If all the light, including the scattered light, is measured, the resulting ratio is called the diffuse transmittance. There are analogous definitions for light specularly reflected from an opaque sheet (specular reflectance) and light directly reflected and scattered from a sheet (diffuse reflectance).
For practical considerations, it is highly desirable for a scanner to measure diffuse transmittance or reflectance rather than specular transmittance or reflectance. Although the specular and diffuse measurements are related, the specular transmittance or reflectance changes a great deal more for a particular sheet than the diffuse transmittance or reflectance. The ratio of the logarithm of diffuse to the logarithm of specular transmittance, called the Callier coefficient, can be as high as 1.5. This means that if the diffuse transmittance is 0.01 the specular transmittance will be 0.001. The much smaller amount of light collected by a specular measurement can lead to substantial difficulties in design of light measurement electronics.
Previous transmittance and reflectance measurement scanners placed the light collection optics close to the film to collect as much of the scattered light as possible. These scanners then moved the light collection optics and light source with respect to the film, as on a drum scanner where the film is wrapped around a drum and then the drum is rotated. The light source and light collection optics are then scanned longitudinally to measure the transmittance throughout the film. While this technique is widely used, it is a slow one since it involves mechanical motion in both directions, and wrapping the film around the drum makes automatic film handling difficult.
The light collection optics of some systems that measure diffuse transmittance use an integrating sphere. This sphere has a small hole through which light enters and further has a light detector placed in the wall away from the direct path of the light. The sphere wall is coated with a very high reflectance material so that substantially all the light incident on the wall is reflected. Thus, regardless of the direction of the light entering the sphere, a fixed fraction of the entering light strikes the detector and is measured. To measure the transmittance with an integrating sphere at many points throughout a sheet, the sphere is physically moved across the surface or the sheet is moved with respect to the sphere.
Because of the drawbacks associated with light-collecting systems used with scanners of the type described, a need exists for improvements in such a system. The present invention satisfies this need.